One conventional method of making acrylonitrile-containing polymers for the production of filaments and fibers begins with an aqueous polymerization process. Following polymerization a filtering process removes most of the water, leaving a wet polymer cake. The polymer cake is then heated to drive off the remaining water and dry the polymer. The dry polymer is subsequently dissolved in a solvent to form a dope which is spun into filaments. Disadvantages of this process are that a substantial amount of energy is required to dry the polymer and that heating the polymer for the purpose of drying can easily degrade the polymer.
U.S. Pat. No. 3,313,758 avoids the drying of the polymer by forming a wet polymer cake and then washing the wet cake with a non-solvent for the polymer to remove a portion of the water and then by washing with a solvent for the polymer for removing more of the water and some of the non-solvent. The disadvantage of this process is that a considerable amount of washing is required and the problem of recovery of the non-solvent is added.
U.S. Pat. No. 3,630,986 avoids the drying of the polymer by mixing a solvent for the polymer with a wet polymer and evaporating off a major portion of the water by the use of a thin film evaporator operated under vacuum. The disadvantage of this process is that the process must tread a thin line between evaporation of the water and high viscosities which will prevent operation of the thin film evaporator. In this process the thin film evaporator must be operated below 100 mm Hg absolute pressure to keep the polymer solution viscosity low enough for operation (below 1,000 poises). This imposes equipment and process limitations.
French Pat. No. 1,266,100 avoids the polymer drying by washing the wet polymer with a solvent such as dimethylformamide to reduce the water content of the polymer. Unfortunately, this process requires several successive washes and thereby multiplies the solvent recovery problem.
One of the objects of this invention is to provide a novel and improved process for preparing spinning solutions of acrylonitrile polymers.
Another object of this invention is to provide a method for making spinning solutions of acrylonitrile polymers obtained from an aqueous polymerization process without the necessity of a drying step.